Crystal structure of saccharopine reductase from Magnaporthe grisea, an enzyme of the alpha-aminoadipate pathway of lysine biosynthesis

Background: The biosynthesis of the essential amino acid lysine in higher f ungi and cyanobacteria occurs via the alpha -amino-adipate pathway, which i s completely different from the lysine biosynthetic pathway found in plants and bacteria. The penultimate reaction in the alpha -aminoadipate pathway is catalysed by NADPH-dependent saccharopine reductase. We set out to deter mine the structure of this enzyme as a first step in exploring the structur al biology of fungal lysine biosynthesis. Results: We have determined the three-dimensional structure of saccharopine reductase from the plant pathogen Magnapor-the grisea in its apo form to 2 .0 Angstrom resolution and as a ternary complex with NADPH and saccharopine to 2.1 Angstrom resolution. Saccharopine reductase is a homodimer, and eac h subunit consists of three domains, which are not consecutive in amino aci d sequence. Domain I contains a variant of the Rossmann fold that binds NAD PH. Domain II folds into a mixed seven-stranded beta sheet flanked by a hel ices and is involved in substrate binding and dimer formation. Domain III i s all-helical. The structure analysis of the ternary complex reveals a larg e movement of domain III upon ligand binding. The active site is positioned in a cleft between the NADPH-binding domain and the second alpha/beta doma in. Saccharopine is tightly bound to the enzyme via a number of hydrogen bo nds to invariant amino acid residues. Conclusions: On the basis of the structure of the ternary complex of saccha ropine reductase, an enzymatic mechanism is proposed that includes the form ation of a Schiff base as a key intermediate. Despite the lack of overall s equence homology, the fold of saccharopine reductase is similar to that obs erved in some enzymes of the diaminopimelate pathway of lysine biosynthesis in bacteria. These structural similarities suggest an evolutionary relatio nship between two different major families of amino acid biosynthetic pathw ay, the glutamate and aspartate families.